This invention relates to the fabrication and access of magnetic memory cells in a magnetic random access memory (xe2x80x9cMRAMxe2x80x9d).
Magnetic Random Access Memory (xe2x80x9cMRAMxe2x80x9d) arrays of the type disclosed in the two above-incorporated U.S. Patents, and depicted in FIGS. 1a-b herein, include an array of magnetic memory cells (e.g., cell 9) positioned at the intersections of wordlines 1, 2, 3 and bitlines 4, 5, 6. Each cell includes a magnetically changeable or free region 24, and a proximate reference region 20, arranged into a magnetic tunnel junction (xe2x80x9cMTJxe2x80x9d) device 8. The principle underlying storage of data in such cells is the ability to change the relative orientation of the magnetization of the free and reference regions by changing the direction of magnetization along the easy axis (xe2x80x9cEAxe2x80x9d) of the free region, and the ability to thereafter read this relative orientation difference. (The term reference region is used broadly herein to denote any type of region which, in cooperation with the free or changeable region, results in a detectable state of the device as a whole.)
More particularly, MRAM cells are written by reversing the free region magnetization using applied bi-directional electrical and resultant magnetic stimuli via its respective bitline and wordline, and are later read by measuring the resultant tunneling resistance between the bitline and wordline, which assumes one of two values depending on the relative orientation of the magnetization of the free region with respect to the reference region. If the free region is modeled as a simple elemental magnet having a direction of magnetization which is free to rotate but with a strong preference for aligning in either direction along its easy axis (+EA or xe2x88x92EA), and if the reference region is, for example, a similar elemental magnet but having a direction of magnetization fixed in the +EA direction, then two states (and therefore the two possible tunneling resistance values) are defined for the cell: aligned (+EA/+EA) and anti-aligned (xe2x88x92EA/+EA).
An ideal hysteresis loop characterizing the tunnel junction resistance with respect to the applied EA field is show in FIG. 2. The resistance of the tunnel junction can assume one of two distinct values with no applied stimulus in region 50, i.e., there is a lack of sensitivity of resistance to applied field below the easy axis flipping field strength +/xe2x88x92 Hc in region 50. If the applied easy axis field exceeds +/xe2x88x92 Hc, then the cell is coerced into its respective high or low resistance state.
Even if the magnetization pattern of the two regions forming the tunnel junction is simple, reversing the direction of magnetization in the free region during writing can actually affect one or both regions in unexpected ways. For example, the reversal of the free region during writing can result in the inclusion of a magnetic vortex or complex magnetic domain walls, pinned by a defect or by edge roughness. Because the junction resistance depends on the dot product mfreemreference averaged over the junction area, inclusion of such complex micromagnetic structures in the magnetization pattern can substantially corrupt the measured tunnel junction resistance during reading.
For example, shown in FIG. 3 is the magnetization pattern in the free region 124 of an MRAM cell formed symmetrically about its easy axis EA in which a complicated wall structure 132 is clearly evident between otherwise acceptable magnetization pattern regions 130 and 134. This overall magnetization pattern was attained from a nominally uniformly magnetized sample (both top and bottom layers originally pointing to the right), for which the easy axis bias was swept from +700 Oe to xe2x88x92700 Oe and back to +700 Oe. The reversal of magnetization evolved to complicated structure 132 as the field was swept from +700 Oe down to xe2x88x9260 Oe. FIG. 4 is a hysteresis loop depicting the relative direction of magnetization versus applied easy axis field for this corrupt sample. The non-square nature of region 150, results in a cell which will not predictably assume either one of its two states upon the removal of the easy axis applied field, is due to the evolution of such complex micromagnetic structures in the cell.
These undesirable magnetic structures decrease the parametric window of operation of the cell at best, or result in a total collapse of the square hysteresis loop necessary for storage at worst. In addition, the presence of such structures can be expected to cause the switching lines required to reverse, or substantially reverse, the free region to increase in size and/or power.
What are required, therefore, are techniques which eliminate such complex micromagnetic structures when changing the state of a magnetic memory cell in an MRAM array.
To overcome the deficiencies of the magnetic memory cells identified above, the present invention relates to, in one aspect, a magnetic memory having first and second crossing conductive lines forming an intersecting region. A magnetic memory cell is positioned at the intersecting region and includes a changeable magnetic region with a magnetic axis along which two directions of magnetization can be imposed, thereby providing two respective states into which the cell is changeable. The cell is changeable into the two respective states according to magnetic stimuli applied thereto via the first and second crossing conductive lines. The changeable magnetic region is formed to be substantially magnetically asymmetrically shaped about its magnetic axis, therefore allowing the magnetization pattern to evolve properly during writing, without the formation of the complex micromagnetic structures discussed above.
The changeable magnetic region of the cell may be shaped as a substantially planar parallelogram about its magnetic axis, with non-right angles in the comers thereof. Alternatively, or in combination, the changeable magnetic region can be magnetically asymmetrically shaped about its magnetic axis via a built-in magnetic anisotropy, possibly using proximate bias regions.
In another aspect, the present invention relates to a magnetic memory having first and second crossing conductive lines forming an intersecting region. As set forth above, a magnetic memory cell is positioned at the intersecting region and has a magnetic axis along which two directions of magnetization can be imposed thereby providing two respective states into which the cell is changeable according to magnetic stimuli applied thereto via the first and second crossing conductive lines. The cell is positioned in the intersecting region such that its magnetic axis is non-parallel to either the first or second crossing conductive line. In one embodiment, the magnetic axis forms an angle of greater than about 5 degrees with either the first or second crossing conductive line.
In addition to the above-discussed techniques of physically imposing an intentional asymmetry about the magnetic axis of the memory cell, the present invention also relates to arranging the applied magnetic stimuli to the cell from the first and second conductive lines such that the magnetic stimuli is applied thereto asymmetrically in accordance with the relative amplitude and/or timing of the magnetic fields applied by each respective conductive line.
In one embodiment, a bias field is applied using the wordline, and the bitline is swept from a low value to a high value while the bias value is applied. In another embodiment, both lines are swept simultaneously from respective low values to respective high values, but at differing amplitudes, e.g., the wordline at 10% of the value of the bitline. This applied stimulus asymmetry has also been shown to allow the magnetization patterns to evolve properly during writing.
Using the above-discussed techniques, i.e., a physical asymmetry, and/or stimulus asymmetry, the magnetization patterns in the free region of the magnetic memory cells can be expected to reverse from one state into another without the formation of undesirable, complex micromagnetic structures. Overall performance of the cell is improved, since the tunneling resistance of the cell will predictably assume one of two values when the applied writing fields are removed.